1. Field of the Invention
The present invention relates to a method of making optical components or elements from CaF2 with parallel (100)- or (110)-oriented optic axes (principle direction) by tempering at elevated temperatures and suitably adapted cooling, The invention also relates to the CaF2 crystals made by the method and to the optical components made from the crystals.
2. Related Art
Calcium fluoride is used as material for optical components for VUV-applications in microlithography, like a wafer stepper or excimer laser. The crystals are the starting material for making lenses, prisms and other components, which are used in optics for imaging the smallest of structures for making integrated circuits, e.g. computer chips. In order to attain the required imaging quality, this optical material has very high specifications. Conventionally the non-uniformity of or variations in the refractive index Δn should be no more than 1×10−6 and the stress birefringence must be clearly below 1 nm/cm.
The stress optical tensor gives the connection between the mechanical variables (such as stress) and the optical effects caused by them (such as the stress birefringence SDB) in crystals (direction-dependent). That means that stresses of equal magnitude in single crystal material can lead to clearly different stress birefringence and non-uniformities in the index of refraction depending on the crystallographic orientation and/or the observation direction. For these reasons up to now components were used in the application or usage direction, in which the minimal stress birefringence is observed. For calcium fluoride crystals that direction is the (111)-direction. Thus currently materials for lens blanks are used exclusively in the (111)-orientation (and/or near the (111)-direction for cubes).
The experiments of J. H. Burnett, Z. H. Levine, E. L. Shirley, described in “Intrinsic birefringence in calcium fluoride and barium fluoride”, Physical Rev. B 64 (2001), 241102 have shown that calcium fluoride has an intrinsic birefringence. This effect strongly increases near the band edge of the material and leads to significant imaging errors using application wavelengths of 157 nm. In order to compensate for the intrinsic birefringence, the objective manufacturer combines lenses of different crystal orientation. Furthermore the lens blanks, rectangular prisms and prisms in general must be made in different crystal orientations, particularly in the (100)-orientation and (110) orientation.
The specifications for not-(111)-oriented products regarding the optical quality, particularly the index of refraction uniformity and the stress birefringence, are comparable with the specifications, which are required of (111)-material. Generally these specifications regarding the quality for not-(111)-oriented products are not equally difficult to attain. The residual stresses in material for (100) products and/or (110)-oriented products are generally about 80 to 90% less than for (111)-oriented products, in order to attain the same stress double refraction or birefringence.
Various possibilities for making single crystals for use as optical elements are known and the principles regarding this use of single crystals are described, e.g., in the textbook Wilke-Bohm, “Crystal Growth (Kristallzüchtung)”, Harri Deutsch Press, ISBN 3-87144-971-7. Single crystals can be made from the gas phase, the melt, solution or a solid phase by diffusion and/or re-crystallization processes according to very different methods.
Suitable CaF2 blanks or semi-finished elements are made in a multi-step process. The prerequisite is to provide a CaF2 powder as starting material, which meets the highest specifications for chemical purity. Traces of critical cations and/or anions may only be contained in sub-ppm amounts or for less crucial purities up to a few ppm. This powder is usually subjected to a drying stage in vacuum. Interfering residual oxygen is removed from the CaF2 by means of an added second material, by a subsequent heating by means of a so-called scavenger reaction. Scavenger substances are, e.g. ZnF2, PbF2 or other suitable fluorides or fluorine-containing gas.
The conventional methods for making calcium fluoride single crystals on an industrial scale include, e.g. the Bridgman-Stockbarger method, the Vertical Gradient Freeze method, the Naken-Kyropoulos method and the Czochralski method. In these methods polycrystalline material is melted in a vessel or crucible. Subsequently the melt is very slowly allowed to solidify in a directed manner, in order to allow the crystals to form. Subsequent cooling must be conducted so that as little as possible thermal stresses, which produce crystal defects, are generated in the crystal.
To fulfill the high specifications for VUV applications the crystals or parts of them undergo a further temperature treatment below the melting point in order to reduce crystal defects and attain a reduced stress birefringence and high refractive index uniformity. During this process characterized as tempering the still-present defects such as dislocations or small angle grain boundaries are clearly reduced at the elevated temperature in the crystal by mechanical deformation and diffusion processes. The subsequent cooling stage entirely determines the obtained quality level.
The tempering can be performed as a process step directly in the crystal growth apparatus or also as a separate process in a special oven.
Typical procedures for tempering calcium fluoride were already described in EP 0 939 147 A2 or in U.S. Pat. No. 6,332,922 B1. Especially special temperature and time conditions are described for improving the stress birefringence and index of refraction uniformity of calcium fluoride crystals. However the described methods do not provide crystals, which have the required quality to fulfill the actual specifications for microlithography with wavelengths of 193 nm and/or 157 nm, which have developed in the meantime.
The (111)-orientation is preferred for blanks or semi-finished articles based on the anisotropy of the optical stress properties according to the disclosures in EP 0 942 297 A2. The (111)-, (100)- and (110)-directions were tested. It was found that only the (111)-disk had stress birefringence values approximately in the required range when CaF2 disks having the different directions were tested under the same heat treatment conditions.
For BaF2 disks it was shown that the attained reduction of the stress birefringence for the (111)-oriented disks was substantially greater than that obtained for disks that are not in the (111)-orientation.
As is the case with CaF2 the required quality cannot currently be attained with an optical principle direction that is different from (111).